Insulin is a protein. Proteins are essential to all forms of life. Proteins perform many different functions within a living body. There are structural proteins, whose function is like that of structural part of a building. Structural proteins form the skeleton in cells that other molecules arrange themselves on. Some proteins are primarily function to generate motion. These are present in muscles, both skeletal muscles and specialized muscle like the heart. Other proteins enhance chemical reactions in the cell. These proteins are called enzymes. Insulin is primarily a signaling protein.
Coordinating a large distributed complex system like a human body requires some form of communication. The human body uses several forms of communication. The nervous system forms one. If you drop a brick on your foot, the feeling of pain is sent to your brain through the nervous system. The nervous system sends, receives, processes, and responds to stimuli quickly, within less than a second. The nervous system encodes signals in electrical pulses that travel along neurons.
The endocrine signaling system forms a second, parallel communication system. The endocrine system is slower and longer lasting than the nervous system. Signals can take minutes to hours to propagate around the body. The endocrine system encodes signals in the form of molecules, which are then released into the body. The molecules are transported around the body, usually through the blood system. Specific receptors (other proteins) exist in different cells and tissues waiting and listening for a signal. When these receptors recognize the presence of the signaling molecule, they alter some cell’s function.
Insulin’s function in the human body is to encode and transmit a signal. Insulin is only produced in a small fraction of the pancreas, in cells called beta cells. These beta cells produce, store and, occasionally, release insulin. Insulin is released into the blood stream primarily in response to increased blood glucose levels. The signal insulin encodes is one that says, “There’s lots of glucose in the bloodstream”.